1. Field of the Invention
The invention relates to a wide-angle eyepiece lens for a telescope such as a monocular and a binocular and, more particularly, to a wide-angle eyepiece lens having an angle of view greater than approximately 60.degree. which makes an eyepiece lens compact.
2. Description of Related Art
One of wide-angle eyepiece lens used for optical instruments such as telescopes and binoculars that is described in Japanese Unexamined Patent Publication No 9-54256 has an angle of view greater than 60.degree. has an eye-relief distance which is referred to an axial distance between an eye point and an apex of an eyepiece lens on the image side suitably long for telescopes of high magnification. A high-powered telescope such as of ten-odd magnifications is not always convenient for every observation circumstance. For example, because the high-powered telescope held by hands is apt to shake during observation due to, a narrow angle of view, there is a demand for a wide-angle eyepiece lens suitable for low-powered telescopes.
In general, such an eyepiece lens for a low-powered telescope has an eye relief distance longer than it needs and/or a large lens diameter, which makes it hard to utilize the eyepiece lens in a low-powered telescope. That is, because a low-powered telescope is equipped with an eyepiece lens whose focal length is longer than that of an eyepiece lens for a high-powered telescope due to the relationship that the magnification is equal to the focal length of a telescope divided by the focal length of an eyepiece lens, it has an unnecessarily long eye relief distance. Taking it into account that a light ray passes an eye point of the eyepiece lens, a long eye relief distance is accompanied by a proportional increase in diameter of the eyepiece lens. If the eyepiece lens having a short focal length described in Japanese Unexamined Patent Publication No 9-54256 which provides properly long eye relief distances at high magnifications provides unnecessarily long eye relief distances when used at low magnifications and puts the eyepiece lens under the necessity of including a large diameter of lens element. For example, it is well known in the art to use as an evaluation value of compactness of a lens a lens diameter ratio (hmax/f) which is referred to a ratio of the distance of a principal ray passing through an eye point at a specified incident angle from the optical axis at the lens relative to the focal length of the lens. The smaller the lens diameter ratio (hmax/f), the more the lens is compact. The eyepiece lens described in Japanese Unexamined Patent Publication No 9-54256 has a lens diameter ratio (hmax/f) of 0.7 for an angle of view of 60.degree., which indicates that the eyepiece lens has a large diameter.
The applicant of this application has proposed a wide-angle eyepiece lens providing a necessary eye relief distance in Japanese Patent Application No. 9-247701.
This wide-angle eyepiece has a large angle of view of approximately 65.degree.. While the eyepiece lens provides balanced distribution of aberrations and in particular reduces aggravation of distortion and is structured by a reduced number of component lens elements to five to six, it has a great lens diameter ratio (hmax/f) of approximately 0.73 for an angle of view of 65.degree., which is always undesirable for compactness of the eyepiece lens. When employing the eyepiece lens for a telescope having low magnifications and a long focal length, it is hard to provide a light and compact telescope.